As described in European Patent application No. 0,010,456, published April 3, 1980, flow-through apertures providing liquid access to a capillary zone or passage are difficult to use if they are circular in shape. The difficulty concerns the tendency of the liquid to not enter the aperture. The smooth cylindrical sidewalls of circular apertures tend to cause a drop of the liquid deposited in the vicinity of the aperture to draw away from the aperture, rather than enter it, unless careful centering is achieved. If careful centering does not occur, the drop circumference does not completely encompass the aperture, but instead intersects it. The surface tension of the liquid in such cases tends to push the liquid away from such circular aperture instead of into it.
The device described in the aforesaid European Application eliminates the problem by the use of special aperture sidewall configurations. These configurations have been found to be very effective and useful in urging the drop to enter the aperture. However, such configurations exclude circular apertures formed by cylindrical sidewalls. Because circular apertures are the simplest to manufacture, it would also be useful to provide a device which allows circular flow-through or ingress apertures to be used even when metering errors occur. However, as noted above, this requires that the drop always encompass the circular aperture.
It will be appreciated that the metering displacement error, that is, the distance between the aperture center and the center of the deposited quantity of liquid, is an important part of the problem. It is impractical to reduce that error to zero by means of metering apparatus and aperture location tolerance control. The only other readily manipulatable variables are the quantity of deposited liquid and the size of the ingress aperture. The quantity of deposited liquid could be increased to insure that the aperture is always encompassed by the circumference of the deposited liquid, as determined by the maximum expected displacement error. However, due to the magnitude of such maximum displacement error such an approach could drastically and unacceptably increase the requisite volume of deposited liquid from the presently preferred level of about 10 .mu.l.
The opposite approach would be to hold the deposited liquid volume constant and reduce the aperture size. This could insure that the aperture is encompassed by the deposited liquid, even when maximum displacement errors occur. However, apertures significantly smaller than about 3 mm diameter have several drawbacks. The surface area contacted by the drop becomes so large, compared to the area of the aperture, that residual liquid tends to remain on the exterior around the aperture, rather than drain into the aperture. Such behavior alters the volume of the liquid that passes through the capillary passage, which in turn can produce errors in the detected analyte levels if such is the end use of the passage. In addition, the large surface contact area that is contacted (compared to the area of the aperture) tends to induce the drop to wander away from a position centered on the aperture.
Thus, for use of circular ingress apertures, a dilemma has existed prior to this invention. Neither increasing the quantity of liquid relative to a fixed size of ingress aperture, nor decreasing the aperture size relative to a fixed quantity of deposited liquid, has appeared to be a satisfactory solution to the reliable use of circular ingress apertures for liquid deposited with a potential displacement error.
(Merely increasing the size of the aperture to encompass the displacement error is not a satisfactory solution because the drop could enter the aperture without contacting the upper surface of the transport passage. In such a case, capillary flow would not initiate.)